The present invention relates to a ball screw used in an environment in which intrusion of foreign matter into the ball nut or the lubricant being washed away from the ball not is liable to cause an insufficiency of lubrication.
Many sealing devices for ball screws are of a type in which a non-contact seal made of rubber or the like is mounted on the end of the ball nut. In this case, however, since there is a clearance between the inner periphery of the seal member and the groove surface of the screw shaft, the effect of suppressing foreign matter intrusion and sealed grease wash-away is decreased depending on the service conditions, sometimes leading to a lowering in the functional capability or the life of the ball screw. Under severe lubricating conditions as in the case where the surrounding environmental temperature is high or the travel stroke or the travel speed of the ball nut (rpm of the screw shaft) is large, the use of the lubricating oil supplied by the sealed grease alone is likely to result in an insufficient amount of lubrication available in the areas of contact between the balls and the screw groove surface.